1. Field of the Invention
The present invention relates to an inverter transformer for use with DC/AC inverter adapted to turn on cathode ray tube or the like for illuminating the back of a liquid crystal display.
2. Description of the Prior Art
In order to give a better understanding of the present invention, description will first be made with reference to FIG. 6 of the accompanying drawings, which illustrates a conventional inverter transformer wherein a primary winding 3 is wound on a cylindrical spool 2a of insulator bobbin 2 having terminals 1 mounted thereto; and a secondary winding 4 is provided under the bobbin 2. Further, a pair of magnetic cores 5 and 6 are disposed in abutting relationship with each other through an insulator sheet 7 and holding the bobbin 2 therebetween from above and below, thus establishing a closed magnetic circuit.
FIG. 7 illustrates distribution of electric field which occurs between two plate-like electrodes 8a and 8b disposed in opposing relationship to each other with a distance d kept therebetween. Let it be assumed that a voltage of V.sub.G volts is applied to the electrode 8b while the electrode 8a is maintained at 0 volt. For the case where the space between the electrodes 8a and 8b is filled with air alone, the electric field occurring therebetween is given by V.sub.G /d as shown by a dotted line. In contrast thereto, for the case where a dielectric body 9 is inserted between the electrodes 8a and 8b, the electric field turns out to be as shown by solid line arrows electric field E.sub.2 (=V.sub.2 /d.sub.2) occurring in the dielectric body 9 has a gentle slope depending on the dielectric constant of the dielectric body 9, while electric fields E.sub.1 (=V.sub.1 /d.sub.1) and E.sub.3 (=V.sub.3 /d.sub.3) occurring in the air-filled portions each represent a steep slope which is increased by an amount corresponding to the decrease in the slope caused in the dielectric body 9.
In the conventional inverter transformer such as shown in FIG. 6, resin such as varnish or the like (not shown) is filled between the primary winding 3 and the bobbin 2 and between the secondary winding 4 and the bobbin 2 in order to increase the withstand voltage. However, it may happen that the resin fails to be completely filled between the primary winding 3 or secondary winding 4 and the bobbin 2 so that bubbles or the like are generated which tend to result in formation of numerous voids. With the electrodes 8a and 8b of FIG. 7 being replaced with the primary winding 3 and secondary winding 4 respectively, the intensity of electric fields occurring in the gap portions between the primary winding 3 and the bobbin 2 and between the secondary winding 4 and the bobbin 2 is increased due to the fact that the primary and secondary windings are close to each other and the dielectric body of the bobbin 2 is interposed therebetween. As a result, corona discharge is liable to occur especially between the secondary winding 4 and the neighboring components such as cores 5, 6, primary winding 3 and etc. which assume a high voltage, thus leading to dielectric breakdown.